Totally agree with everyone else. A3 with work presented well using the plastic pockets. If you can get one with pockets on the inside cover that'd be useful too, especially as your career progresses. You'll may find yourself working on material such as brochures which you want to keep intact and pull out for the interviewer (or for whoever) to flick through. I've always found this to work well for me.
I actually got my A3 portfolio at a bargain price on ebay a while ago. It looks professional, has the sleeves, a cool strap and those handy pockets. Being A3 it's perfect in size. In fact when I left a previous design job they gave me a leather bound A2 portfolio case for a going away pressie. Have to admit in the 4 years since, I still haven't used it because it's too big.